Solid state ionic polishing of diamond

ABSTRACT

A process and apparatus for polishing diamond or carbon nitride. A reaction and polishing take place at the interface between an oxygen superionic conductor (yittria-stabilized zirconia) and the diamond or carbon nitride. Oxygen anions are transported to the interface under the influence of a chemical gradient and react with the diamond or carbon nitride. Other mechanisms, such as an electric field and/or heat, that increase the partial pressure of oxygen on the opposing side of the interface of reaction can be used to accelerate the reaction time. The process may be undertaken at low temperatures and without mechanical motion, making it an attractive and useful polishing method. In addition, there is no residue of the polishing process which needs to be removed and polishing can be accomplished in ambient air.

TECHNICAL FIELD

This invention relates generally to polycrystalline diamond films, andspecifically to an apparatus and method for solid-state ionic polishingof diamond.

BACKGROUND ART

The deposition of polycrystalline diamond films is a well establishedtechnology, but there still remain several areas which need to beaddressed: growth rate, heteroepitaxy, minimizing graphitic component,and control of the surface roughness, to name a few. The surfaceroughness associated with the growth for polycrystalline diamond remainsa problem for several areas, e.g., heat management, optical andtribiological applications. Different methods have been employed to helpsolve this problem with varying degrees of complexity. One of thesimplest methods has taken advantage of the inability of diamond towithstand machining of ferrous materials such as Fe or Ni. Under certainconditions of heat and pressure carbon tends to diffuse into the metalwhich is in contact with the diamond, forming a carbide layer which ispolished away. (See Want et al., SPIE Diamond Optics III 1325:160(1990); and Yoshikawa, M., SPIE Diamond Optics III 1325:210 (1990)).However, the time needed can be on the order of hours to several weeksdepending on the conditions of temperature, ambient atmosphere, andprior surface treatment. The most efficient polishing is done at hightemperatures (in the 750° to 950° C. range), thereby precludingpolishing in anything but a vacuum or non-oxygen environment.

Other methods that have been investigated are planarizing with a laser(see Yoshikawa, M., SPIE Diamond Optics III 1325:210 (1990)), etchingwith oxygen and argon ion beams using a planarizing layer spun onto thediamond (see Tianji et al., SPIE Diamond Optics III 1325:210 (1990)),and more recently the diffusion of carbon from the diamond into Fe or Mnduring a high temperature (about 900° C.) anneal followed by an acidetch to remove the carbonaceous layer (see Jin et al., Appl. Phys. Lett60:1948 (1992)). In the latter case a mechanical polish was alsonecessary to completely remove the residue left after the anneal.

To minimize complexity, a polishing method using a moderately lowtemperature that can be carried out at atmospheric pressure, without thenecessity of special gases and mechanical motion, is desirable.

DISCLOSURE OF THE INVENTION

The present invention is directed to an apparatus and a method forpolishing diamond or a carbon nitride films which circumvents the needfor high temperatures or special ambient atmospheres and, in addition,eliminates the need to mechanically move the diamond with respect to thepolishing lap.

The method according to the present invention is a static process whicheliminates the necessity to mechanically move the diamond and polishinglap with respect to each other. No pre- or post-processing is required.In addition, there is no residue of the polishing process which needs tobe removed and the polishing can be accomplished at much lowertemperatures and in air.

According to the present invention, diamond or a carbon nitride filmscan be polished by using an oxygen superionic conductor. The superionicconductor may transport oxygen ions to the surface via one or moremechanisms, including a chemical gradient which may be aided by anapplied potential (on the order of 100 V) and/or an elevated temperature(about 250° C. to about 350° C.). Although Applicants are not bound byany particular theory, it is believed that elevated temperatures assistin the reaction of the oxygen with the diamond or a carbon nitridesurface. The diamond or a carbon nitride surface features in contactwith the superionic conductor are volatilized into CO and/or CO₂,resulting in their removal. As a result, the diamond or a carbon nitridesurface takes on the same surface finish as that of the superionicconductor. Embossing and selective etching of diamond or a carbonnitride can also be achieved according to the present invention.

The foregoing and other features and advantages of the invention will beapparent from the following more particular description of preferredembodiments of the invention, as illustrated in the accompanyingdrawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a representative schematic diagram of a polishing apparatus ofthe present invention.

FIGS. 2A, 2B, and 2C show scanning electron microscope micrographs ofunpolished diamond, partly polished diamond, and polished diamond,respectively.

FIGS. 3A and 3B show surface profilometer traces of unpolished andpolished diamond films, respectively, with a stylus radius diameter of12.5 μm.

FIG. 4 shows transmission from 200 to 900 nm for unpolished diamond( - - - ), silicon carbide (SIC) ( . . . ), and polished diamond (-)membranes of 1.04, 1.0, and 1.14 μm thickness, respectively.

FIG. 5 shows a representative diagram of an apparatus for forming carbonnitride films that can be polished according to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Yittria-stabilized zirconia (YSZ), Y₂ O₃.ZrO₂, is one of a class ofmaterials known as defect stabilized ceramic oxide superionicconductors. Other superionic conductors which may be used in the presentinvention include M₂ O₃.ZrO₂. where M=La, Sm, Y, Yb, or Sc; and CaO.MO₂.where M=Zr, Hf, Th, or Ce. (See B. C. H. Steele, Oxygen ion conductorsand their technological applications, Materials Science and Engineering,B13 (1992) pp. 79-87.)

These unique materials are electrical insulators, but at elevatedtemperatures, O²⁻ can be conducted through these materials withconductivities approaching 10⁻¹ (Ω-cm)⁻¹. The motion of anions in thesematerials is associated with the formation of oxygen vacancies due tothe substitution of lower valence cations into the host cationpositions. The level of conduction is generally a function of thetemperature and the amount and speed that O²⁻ will transpire through aparticular material.

It is known that substitution of aliovalent cations of lower valenceinto a higher valence site can result in either cation interstitials oranion vacancies to maintain charge neutrality. (Aliovalent ionsubstitutions are ionic substitutions into the host lattice with acharge that is different that the host and produces defects such asvacancies resulting in charge neutralization.) For the case of Careplacing Zr in CaO.ZrO₂, one anion vacancy is created for every Casubstitution: Zr_(1-x) Ca_(x) O_(2-x) □_(x), where □ represents a anionvacancy. In the case of YSZ, substitution of Y into the Zr sites createone oxygen anion vacancy for every two Y cations. The activationenergies for the defect motions are about 18 to 25 kcal/mol in YSZ (seeChoudhary et al., in Solid Electrolytes and their Applications, p. 42,Subbarao, Plenum Press, New York (1980)).

Turning now to the present invention, an apparatus 100 for performingsolid-state polishing of diamond is shown in FIG. 1. The apparatus 100comprises a polished single crystal piece of 200-1000 μm thick YSZ wafer102 (preferably about 500 μm thick; having Miller indices of (200)), ontop of a polycrystalline diamond film (layer) 104 grown by conventionalmicrowave chemical vapor deposition (CVD) on a crystalline Si substrate106 (between about 300-800 μm thick). The YSZ wafer 102 will hereafterbe called "polishing layer" 102. In a preferred embodiment, the YSZpolishing layer 102 composition is about 9.5 mol % Y₂ O₃ in ZrO₂, whichcorresponds to a high O²⁻ conductivity composition (see Choudhary etal., in Solid Electrolytes and their Applications, p. 41 Subbarao,Plenum Press, New York (1980)). This specific composition is notcritical to the present invention.

Layering (e.g., contacting) of the YSZ polishing layer 102 on thediamond layer 104 permits elevated carbon features on the diamond toreact with O²⁻ from the YSZ layer 102 causing a chemical gradient toform within the layers. If the partial pressure of oxygen on the uppersurface of the YSZ polishing layer 102 is increased to about 0.2-1.0atm, additional O²⁻ diffuses through the layer 102 to the interface withlayer 104 for reaction with the carbon and polishing of the diamond. Asthe carbon surface features react with O²⁻, the diamond layer 104 is ineffect polished. However, the partial pressure of oxygen may be betweenabout 0.001 to 100 arm.

The apparatus 100 may include an optional heater means used to elevatethe temperature of at least layers 102 and 104 to achieve a temperatureof about 25°-950° C., most preferably, about 390° C. The heater meansmay comprise an electrically heated block 108, e.g., a conventional hotplate, or the like. In the alternative, the heater means may be a meansfor providing electromagnetic radiation which can be absorbed by one ofthe layers 102 and 104, e.g., microwave, laser or RF energy. Theelevated temperature acts as a further activation source which allowsthe O²⁻ to overcome the energy barrier present between the anion sitesin the defect structure of the YSZ polishing layer 102. Thus, when aheater means is employed, the diamond layer 104 may be polished at afaster rate.

The apparatus 100 may further include an optional means 110 for applyingan electrical field gradient, or potential, across at least layers 102and 104 which drives the O²⁻ toward the diamond surface. In a preferredembodiment, the means 110 may comprise a conventional current-limitedpower supply having a positive terminal connected to the diamond layer104 and/or the substrate 106 at a contact 112, and a negative terminalconnected to the upper surface the YSZ polishing layer 102 (i.e., thesurface opposite the surface adjacent the polishing interface). Thecontact 112 can be made by coating with suitable metal or conductiveoxide materials, such as Pt films from about 50-1000 521 , and/orconducting oxides such as pervoskite oxides with the general formula ofABO₃ or mixed oxide conductors such as ABO₃ -ABO₂.5 (as described inSteele, supra). Attached to the contacts 112 and 114 are conductors thatprovide current paths to the power supply 110.

Because YSZ is an electrical insulator, the contact 114 may be a pointcontact giving rise to a very non-uniform electric field, with the fieldlines being more concentrated near the point of contact. The non-Uniformelectric field can be eliminated if the surface of the YSZ polishinglayer 102 opposing the polishing surface is coated with a conductivelayer 116 comprising a paste, paint, or the like. In this case, thecontact 114 is made to the conductive layer 116. A conductive paste ofgraphite, silver, platinum, or an equivalent electrically conductivematerial, capable of transpiring oxygen, may be applied to the uppersurface of the YSZ polishing layer 102 and sintered as necessary toimprove the even spreading of the electric field. Alternatively, dopingor ion implantation can be used to increase the conductivity of theupper surface of layer 102.

Single crystal or polycrystalline ionic conductors can be used, withonly a small reduction in ion conductivity expected for the latter.Again, to improve the uniformity of the electric field through the ionicconductor, a permeable electrode (e.g., about 50 Å of Ag or Pt, or othersuitable metal) can be deposited onto its surface. To implementsolid-state ionic polishing on the surfaces of non-conductive materials,one can use another technique to change the surface conductivity to thedesired depth of polishing. This may take the form of a damage layer viaimplantation or laser processing.

Initial voltages of between about 100 to 200 V may be applied and heldconstant for several hours, with resulting currents of several to tensof milliamperes. By employing the current-limited power supply, acontrolled etch can be achieved with typical operating parameters of 1mA at 50 to 80 V. The 1 mA applied current may be chosen as the standardpolishing condition. Variations in the size of the YSZ polishing layer102 will result in changes in the actual current density therethrough.

The O²⁻ is formed in the superionic conductor layer by the formation ofordered vacancies in the layer. The O²⁻ is then transported by thechemical gradient to the diamond YSZ interface. The mechanisms forpolishing is believed to be the formation of a volatile oxide of carbon,e.g., CO and/or CO₂ at the diamond-YSZ interface. Because the freeenergy of formation for CO₂ (-94.3 kcal/mol) is lower than that of CO(-32.8 kcal/mol), CO₂ is believed to be the dominate volatile product ofpolishing.

The following initial polishing rate (on rough samples about 1 μm inthickness) has been observed by the inventors: 1940 Å in 30 min (about65 521 /min) using a fixed current of about 25 mA applied to an area ofabout 0.14cm² (which is about 1.8 mA/cm²). If the O²⁻ ions are the solecharge carriers, the flux of O²⁻ through the YSZ polishing layer 102 canbe related to the current density by i=2 eJ, where i=current density; eis the electron charge; and J is 5.6×10¹⁵ O²⁻ /cm² -Sec.

Assuming a diamond film density of 3.5 g/cm³ (bulk), this corresponds toan etch rate of about 191 Å/min for a smooth diamond film, providingthat each O²⁻ reacts to form a molecule of CO. The apparent processefficiency, or carbon atoms removed per O²⁻ ion, is thus 0.34. However,the etch rate of rough diamond are expected to be higher, because theyhave a smaller diamond/superionic conductor contact area (implying ahigher local O²⁻ flux at the facet contacts), and because polishing onlyremoves material form the facets (so the same etch depth can be achievedwith the removal of a smaller number of carbon atoms). The similarity ofthe observed and calculated diamond etch rates thus suggests that theoverestimate of the process efficiency resulting from the assumption of100% reaction of O²⁻ with carbon is compensated by the higher initialetch rate on the rough sample. The influence of film topology on theetch rate is also suggested by the observation that the apparent processefficiency, again averaged over a 30 min etch time, decreases at highercurrent densities (e.g., an efficiency of about 0.13 for i=14.3 mA/cm²).

Assuming that CO₂ is the reaction product, this corresponds to a removalof about 1.5×10¹⁵ carbon atoms/cm² -sec, or approximately 1monolayer/sec. Given a covalent radius of about 0.77 Å for a carbonatom, this corresponds to about 1.5 Å/sec removal rate. Tests on a 0.14cm² piece of YSZ with a 0.25 A applied current (1.8 mA/cm²) for about20-40 mins resulted in removal of about 1940 Å of diamond. This compareswith a predicted removal of 4860 Å, a factor of 2.5 less. The loweramount of diamond etched away may be attributed to the followingfactors: (1) not all O²⁻ reacting with the carbon to form CO₂ ; (2)initially the YSZ is only in contact with the peaks of the diamondfacets, so that the effective area for the reaction to take a place isinitially small; (3) O²⁻ conduction may not be the only mechanism; therecould be cation, electron, or hole conduction occurring resulting in alower than expected O²⁻ transport. All of these factors are at presentbeing addressed in the following ways.

To determine what the reaction product is, a correlation of current flowin the YSZ with the output from a residual gas analyzer, will givedirect evidence of which oxide of carbon is being produced. Startingwith smooth diamond will eliminate the concern of contact area so that atrue rate can be established. To address the mechanism of O²⁻conduction, impedance spectroscopy measurements may be conducted todetermine if there are any other charge carriers participating under thepolishing conditions. This possibility is most unlikely if one considersthat Arrhenius plots of conductivity versus 1/T for 10 mol % Y₂ O₂ inZrO₂ shows linear behavior down to 400° C. (See Catlow, C.R.A, inSuperionic Solids and Solid Electrolytes: Recent Trends, p. 366, ed.Laskar et al., Academic Press, New York (1980).) This is indicative ofO²⁻ vacancy motion dominating the conductivity (see Etsell and Flengas,Chemical Rev. 70:339 (1970)).

Under high current and limited oxygen conditions, the YSZ polished layerdarkens throughout its thickness. This has been observed by others (seeCasselton, R. E. W., in Electromotive Force Measurements inHigh-Temperature Systems, p. 157, ed. Alcock, A. C., American ElsevierPublishing Co., New York (1968)), and is attributed to the formation ofcolor centers. These color centers are formed as a result of oxygenanions being depleted from the YSZ polishing layer at a rate faster thanthey can be supplied by the ambient atmosphere via the reaction 1/2O₂→O² +V_(o), where V_(o) =anion vacancy. The excess vacancies formed arethen filled by electrons from the negative terminal, thereby causing thedarkening.

At the operating conditions of 1 mA in air, no darkening is observed.Controlled experiments with applied currents up to 3 mA (about 21.4mA/cm²) for 30 minutes in air have shown no evidence of darkening. Thedarkening only occurred under controlled polishing when the appliedcurrent was not regulated and typically exceeded 120 mA, which wouldcorrespond to greater than about 100 mA/cm² for 30 minutes some cases.If the ambient atmosphere in which the polishing is taking place is madeoxygen rich, a higher current density can be tolerated because of theincrease in the ion current transport (see Steele, supra). An upperlimit of current density was not established for the present process.

FIGS. 2A-C depict three scanning electron microscope (SEM) micrographsshowing the initial diamond surface and two diamond layers polished todifferent stages of completion, respectively. The very faceted surfacemorphology is most apparent in FIG. 2A. Note that in FIG. 2C the grainstructure of the polycrystalline diamond layer is still visible afterpolishing away the facets and the etch appears to be non-selective(i.e., non-orientation dependent).

FIGS. 3A and 3B show surface profilometer traces of unpolished andpolished diamond films, respectively, with a stylus radius diameter of12.5 μm. FIG. 3A shows that the peaks of the facets have been polishedflat. FIG. 3B shows another layer which has been polished further thanthat in FIG. 3A. Note that the typical peak-to-valley distance is about400 Å for the unpolished diamond, whereas the peak-to-valley for thepolished diamond is less than about 50 Å. A substantially smoothsuperionic conductor layer (e.g., typical peak-to-valley distance ofless that about 20 Å), can be used to achieve substantially smoothdiamond films in accordance with the present invention.

FIG. 4 shows the percentage transmission of electromagnetic radiationfrom 200 to 900 nm for unpolished diamond (- - - ), high temperaturechemical vapor deposition (HTCVD) silicon carbide (SIC) (. . . ), andpolished diamond (-) membranes of 1.04, 1.0, and 1.14 μm thickness,respectively. FIG. 4 shows optical transmission in polished diamondbegins to fall off below about 225 nm, and still exhibits transmissionof about 9% at 200 nm. This compares with the near-UV cut-off of about226 nm for natural type IIa diamond (see Field, in The properties ofDiamond, p. 652, ed. Field, Academic Press, New York (1979)). Thefall-off is indicative of very little nitrogen impurity in the film,unlike type Ia diamond which has a strong UV absorption and cut-off atabout 246 nm (see Field, in The properties of Diamond, p. 652, ed.Field, Academic Press, New York (1979)). This suggests thatpolycrystalline diamond polished according to the present invention haseven less nitrogen than type IIa. Transmission measurements out to 2500nm show little difference between the polished and unpolished diamond,and only start to deviate from each other at wavelengths below about1000 nm. At these wavelengths the surface roughness starts to dominateand transmission is degraded.

In further embodiments of the present invention, a range of oxygen ionconductors can be used, i.e., defect stabilized ceramic oxides of theclasses: CaO.MO₂, where M=Zr, Hf, Th, Ce and M₂ O₃.ZrO₂, where M=La, Sm,Y, Yb, and Sc. Any other O²⁻ ion conductor can also be employed, withits efficiency as a polishing agent determined by the flux of anionswhich can be conducted through the material per unit electric field. Thepresent invention is also applicable for polishing carbon or anycarbonaceous material that reacts with oxygen and any O²⁻ ion conductor,such as hard materials, e.g., C_(x) N_(y), whose oxides are volatile.

C_(x) N_(y) films have been polished using the same conditions used topolish the polycrystalline diamond films described above. For example,carbon nitrides have been formed via an ion bean sputtering techniqueemploying Kaufman-type and microwave driven ion bean sources.

A representative apparatus 500 for forming carbon nitrides is shown inFIG. 5. A Kaufman-type broad-beam nitrogen ion source 502 is aimed at agraphite target 504 and the sputtered atoms are collected on one or morea substrates 506 mounted as generally shown in the figure. In addition,a second nitrogen ion bean source 508, which can be either aKaufman-type or a microwave driven ion bean source, is aimed at thesubstrate 506 during the deposition in order to incorporate a higherfraction of nitrogen into the films being grown. The composition of theresulting films (C_(x) N_(y)) can be varied through control of the ionenergy and ion current density from the second source 508 aimed at thesubstrate 506 during film growth. The minimum value that has beenobtained for x according to this technique is 1.2 (i.e., C₁.2 N₁).

Compositional analysis was made using Rutherford BackscatteringSpectroscopy (RBS) employing aluminum precoated carbon substrates toseparate the film and substrate signals for analysis. X-ray diffractionanalysis of the films revealed an amorphous state in all cases.Electrical characterization was performed using a standard four-pointprobe. Film depositions were carried out at temperatures ranging fromambient to 700° C. with the film resistivity ranging from 3×10⁻³ ohm-cm(at a deposition temperature of about 656° C.) to about 5 ohm-cm (at adeposition temperature of about 30° C.).

In a preferred embodiment of the present invention, apparatus 100 ofFIG. 1, without means 110, is placed in a low pressure plasma CVDchamber. A process gas of oxygen at about 0.001-1.5 atm is convertedinto a plasma in a conventional manner, which supplies ionized oxygenO²⁻ to the YSZ polishing layer 102. The oxygen plasma is at the oppositesurface of the YSZ polishing layer and the YSZ polishing layer is inbetween the plasma and the diamond. A potential can also be sustained onthe plasma, which is different than the potential at the interfacesurface. Thus, the plasma is not only a chemical source of O²⁻, but alsomay form one of the "electrodes" for an electrical potential gradient.Thus an electric potential gradient may be provided in the CVD chamberto promote further transpiration of oxygen through the layer 102.Controlled heating of the chamber can also be performed in a straightforward manner. A source of confirmation is The Handbook of PlasmaProcessing Technology, Edited by S. M. Rossnagel, J. J. Cuomo and W. D.Westwood, Noyes Publications (1990).

The solid-state ionic polishing of the present invention may be used topattern valleys in an etched ionic conductor with a metal which acts asan etch stop for diamond embossing. Embossing of diamond layers isapplicable to archival storage, as well as patterning of diamond coatedstampers for compact disks's, and other applications where a patterneddiamond layer is desired.

Diamond films having substantially scratchless polished surfaces,compared to conventionally polished films, can be produced in anatmosphere having oxygen by using a substantially scratchless surface ofa superionic conductor layer. By using a patterned superionic conductorlayer, the polishing transfers a negative imprint of the pattern to thediamond film.

The present invention produces smooth diamond surfaces for manyapplications, such as: reduced optical scatting, a planar surface forlithography, planar surface for tribological application, planar surfacefor thermal management applications and to improve the thermalconductivity of the surface. The present invention may be employed inmany other applications where a smooth diamond surface is necessary.

Although no mechanical motion is necessary for solid-state ionicpolishing according to the present invention, which defines one of theunique properties of the disclosed method, the solid-state ionicpolishing may be enhanced in combination with mechanical motion so as toprovide a chemical-mechanical polish.

While the invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that the foregoing and other changes in formand details may be made therein without departing from the spirit andscope of the invention. All cited patent documents and publications inthe above description are incorporated herein by reference.

Having thus described our invention, what we claim as new and desire tosecure by Letters Patent is:
 1. A method for polishing a diamond film ora carbon nitride film, comprising the step of:(1) contacting the film tobe polished with a surface of a superionic conductor layer to form aninterface; and (2) supplying said superionic conductor layer withoxygen, wherein said superionic conductor layer transports the oxygensuch that the oxygen reacts with carbon in the film, thereby polishingthe film.
 2. The method of claim 1, wherein said oxygen is present at apartial pressure of about 0.001 to 100 atm.
 3. The method of claim 2,wherein in step (2) said polishing is performed in a plasma chemicalvapor deposition chamber, and the partial pressure of the oxygen isabout 0.001 to 1.5 atm.
 4. The method of claim 1, wherein in step (2),said superionic conductor layer and the film are heated at a temperatureof about 25° to 650° C.
 5. The method of claim 1, wherein in step (2),an electric field is applied across at least said superionic conductorlayer.
 6. The method of claim 1, wherein said surface of said superionicconductor layer is patterned, and said polishing thereby transfers anegative imprint of said pattern to the film.
 7. The method of claim 6,wherein the film is a single crystal diamond film.
 8. The method ofclaim 6, wherein the film is a polycrystalline diamond film.